1. Field of the Invention
The present invention relates to a toner for developing static charge images in electro-photography, electrostatic recording, and electrostatic printing, and a developer, a toner container, a process cartridge, an image-forming apparatus, and an image-forming method, using the toner.
2. Description of the Related Art
Generally, in electro-photography, images are formed by a series of processes including forming a static charge image on a photoconductor (a static charge image bearing member), developing the static charge image using a developer to form a visible image (a toner image), and transferring and fixing the visible image to a recording medium such as a sheet of paper to form a fixed image (see the U.S. Pat. No. 2,297,691). The remaining toner that is not transferred to the recording medium is cleaned off by a cleaning member such as a blade pressed against the photoconductor surface.
For developers, one-component developers using alone a magnetic or non-magnetic toner and two-component developers comprising a toner and a carrier are known. Toners are usually produced by a kneading and pulverizing method in which a thermoplastic resin is melted and mixed with a pigment, a releasing agent such as wax, and a charge controlling agent, pulverized, and then classified. Fine inorganic particles or fine organic particles are added to the surface of toner particles for improved flowability and cleaning ability where necessary.
The toner produced by the aforementioned kneading and pulverizing method generally has wide particle diameter distribution and easily experiences non-uniform frictional charges and, therefore, fogs. Considering production efficiency, it is difficult to obtain smaller toner particles having a volume average particle diameter of 2 μm to 8 μm. Therefore, demand for high image quality is not satisfied.
Toners granulated in an aqueous phase are a focus of interest. Such toners have a narrow particle diameter distribution, are easily made smaller in particle diameter, allowing for high quality and highly fine images, and are excellent in terms of offset resistance due to highly dispersed releasing agents and in fixing properties at low temperature. Being uniformly charged, the toner has excellent transfer properties, and also has excellent flowability. It is advantageous in designing a hopper and a developing apparatus because a smaller torque is required for rotating the developing roll.
Prior art methods for granulating the toner in an aqueous phase involve polymerization or emulsification and dispersion. The toners obtained by these methods (occasionally referred to as the chemical toner, hereafter) have been developed.
A variety of polymerization methods are known, and include a well known suspension polymerization method, in which monomers, a polymerization initiator, colorants, and a charge controlling agent are added and stirred into an aqueous phase containing a dispersion stabilizer to form oil droplets, which is heated for a polymerization reaction to obtain toner particles. An association method is proposed in which fine particles are formed by emulsion polymerization or suspension polymerization, the fine particles are aggregated, and the aggregated fine particles are allowed to fusion bond to obtain toner particles.
The toners obtained by the aforementioned polymerization or association method can have a smaller toner particle diameter. However, the main component of their binder resin is limited to radical-polymerizable vinyl polymers and, therefore, the use of polyester and epoxy resins that are preferable for color toners is not allowed. The polymerization method also has the problem of difficulty in reducing VOCs (volatile organic compounds comprising un-reacted monomers) and to obtain a toner with a narrow particle diameter distribution.
The emulsification and dispersion method is a method in which a mixture of a binder resin, colorants, and other components is mixed with an aqueous phase and emulsified to obtain toner particles (see Japanese Patent Application Laid-Open (JP-A) Nos. 5-66600 and 8-211655). Like the polymerization method, the toner can easily have a smaller particle diameter and a spherical particle shape. In addition, this method has the advantages of having more options for a binder resin compared to the polymerization method, ease of reducing residual monomers, and colorants and other components can be used at any concentration, from low to high.
It is preferable that a binder resin be fixed at low temperature and rapidly melt during fixation to make the image surface smooth. For example, polyester resin is more preferable than styrene-acrylic resin. Particularly, highly flexible polyester resin is preferable for color toners. Recent focus has been given to an emulsification and dispersion method for producing a small particle toner containing polyester resin as a binder resin. Such a toner cannot be produced by the aforementioned polymerization method.
However, toner produced by the emulsification and dispersion method also fails to allow for low fixation temperatures and wider offset resistance temperature ranges. In addition, the creation of fine particles during the production process and some emulsification loss are inevitable, which reduces the toner yield and, therefore, the productivity.
In order to resolve these problems, a method is proposed in which, after emulsification and dispersion, polyester resin is used as a binder resin, the obtained fine particles are aggregated and bonded by fusion to produce toner particles (see JP-A Nos. 10-020552 and 11-007156). This method does not produce superfine particles. Hence, there is no emulsification loss, or toner having a sharp particle diameter distribution and, therefore, no classification can be obtained. Used polyester resin mainly has a straight-chain structure or a low viscosity. Low temperature fixing properties and high temperature offset resistance cannot be simultaneously obtained, making the toner unsuitable for recently desired oil-less heat roll fixation.
Such chemical toners essentially have a spherical shape because of the interfacial tension of oil droplets that occurs in the dispersion process. The spherical toner particles are flowable even if they are of small particle diameter. This is advantageous in designing a hopper and a development apparatus because a smaller torque is required for rotating the developing roll. On the other hand, it is difficult for some cleaning systems to clean them off. The photoconductor surface is cleaned by a unit such as a blade, a fur brush, or a magnetic brush after a toner image is transferred. Among these, blade cleaning is generally used because it has a simple structure and high cleaning ability. In blade cleaning, spherical toner particles are rotated and infiltrate between the cleaning blade and photoconductor, which makes cleaning difficult.
For applying chemical toners to blade cleaning, a method is proposed in which high speed stirring is conducted before the polymerization ends, applying a mechanical force to the particles so as to give the polymerized particles irregular shapes (see JP-A No. 62-266550). However, this method disturbs a stable dispersion state and accelerates association between particles, potentially leading to a mass of polymers. It is difficult to control stirring, making the method impractical.
Alternatively, for example, a method is proposed in which polyvinyl alcohol having a specific saponification value is used as a dispersant to aggregate particles to associated particles of 5 μm to 25 μm for improved cleaning ability (see JP-A No. 2-51164). However, the associated particles are easily grown to larger particle diameters using this method. Therefore, it is not suitable for producing small-particle toners.
In another proposed method, irregularly shaped particles are formed by the phase reversal emulsification, followed by removal of organic solvent, which is stopped mid-way, aggregation, and fusion bonding (see JP-A No. 2002-351139). This method requires a self-emulsifying resin and has significant limitations on the type and acid number of resin, which leaves few options for materials. Controlling the shape by stopping the removal of organic solvent mid-way requires many steps of fine adjustments and controls, increasing costs in view of facility and productivity. In practice, this method is not suitable for mass production.
Hence, in fact, a toner having irregularly shaped particles for excellent cleaning ability (for example, not suffering from poor blade cleaning) and for high image quality while maintaining the advantages of chemical toners such as small particle diameters, narrow particle diameter distributions, and flowability has not been provided.